1. Field of the Invention
The present invention relates to a wavelength selective switch which separates a wavelength division multiplexed light for each wavelength by a spectral element, and thereafter, condenses the separated lights using a lens to reflect the condensed lights by movable mirrors, to thereby switch paths for the lights of respective wavelengths, and in particular, to a wavelength selective switch by which the connection between an input side and an output side is blocked at a non-operating time.
2. Description of the Related Art
In a photonic network of which connection between nodes is a ring type or a mesh type, there has been adopted a structure in which, in the case where a failure occurs in a certain node or a certain path, in order to avoid the degradation of communication service over the entire network, an optical signal is transmitted via a node or a path in which a failure does not occur, to improve the communication reliability. Further, when the node or the path is restored from the failure, the setting of an optical switch in each node is made based on a previously determined rule. Therefore, in the optical switch which is in a non-operating state due to any apparatus failure, if the path between an input side and an output side is connected, there is caused a problem in that the network is unintentionally connected. In order to solve such a problem, for the optical switch on each node, it is required that the connection between the input side and the output side is blocked at the non-operating time.
As one of optical switches capable to be arranged on each node of the photonic network as described above, there has been proposed a wavelength selective switch by which optical directions are switched using movable mirrors (refer to U.S. Pat. No. 6,661,948).
FIG. 12 is a perspective view showing a configuration example of a conventional wavelength selective switch. This conventional wavelength selective switch comprises: a fiber collimator array 101; a diffraction grating 102; a condenser lens 103; a mirror array 104; and a ¼ wave plate 105. In the fiber collimator array 101, N (≧3) fiber collimators are arranged in one direction, to make up one input port and a plurality of output ports. A WDM light output from the input port is separated to different angle directions, according to wavelengths, by the diffraction grating 102, and thereafter, the lights of respective wavelengths are condensed on different positions by the condenser lens 103. On condensing positions of the lights of respective wavelengths, there is arranged the mirror array 104 provided with a plurality of movable mirrors corresponding to the number of wavelengths. Each movable mirror is a micro-mirror which is formed using, for example, a Micro Electro Mechanical Systems (MEMS) technology and an angle of a reflecting surface thereof is controllable according to a drive signal. The lights of respective wavelengths reached the mirror array 104 are reflected respectively by the movable mirrors corresponding thereto, to be turned to directions according to the angles of the reflecting surfaces of the movable mirrors. At this time, the reflecting surface of each of the movable mirrors is controlled to a predetermined angle corresponding to a position of any one of output ports, which is set as the output destination of a light to be input, so that the lights of respective wavelengths turned by the movable mirrors pass through the condenser lens 103, the ¼ wave plate 105 and the diffraction grating 102, to be led to the destination output port.
FIG. 13 is a perspective view showing another configuration example of the conventional wavelength selective switch. This wavelength selective switch has a configuration same as the above configuration example shown in FIG. 12 except for that a transmission type diffraction grating 102′ is used.
Incidentally, herein, a direction at which the lights of respective wavelengths are angularly dispersed by the diffraction grating is an X-direction, a direction in which the input and output ports are arranged is a Y-direction, and an optical axis direction vertical to an X-Y plane is a Z-direction.
Such a conventional wavelength selective switch has a wavelength selecting function capable of controlling the angle of the reflecting surface of each movable mirror on the mirror array 104 to select the light of arbitrary wavelength from the lights of a plurality of wavelengths contained in the input WDM light, to thereby lead the light of arbitrary wavelength to the destination output port. Further, it is also possible to reverse a relation between the input and the output, so that one of the lights supplied to a plurality of input ports is selected to be led to one output port.
In the conventional wavelength selective switch as described above, for blocking the connection between the input and output ports at the non-operating time, as shown in a top view of FIG. 14 and a side view of FIG. 15 for example, it can be considered that the mirror array is mounted in a state where the angle of each movable mirror at the non-operating time is intentionally offset to a direction for switching the ports (Y-direction) from a state where the connection between the input and output ports is optimum. FIG. 14 and FIG. 15 show the example in which the transmission type diffraction grating shown in FIG. 13 is used, but for the reflective type diffraction grating as shown in FIG. 12, same consideration can be made. Thus, by offsetting the angle of each movable mirror to the Y-direction, the lights which are reflected by the movable mirrors and thereafter reach the collimator array are deviated to the Y-direction at the non-operating time, and therefore, the connection between the input and output ports is blocked. Then, when the connection between the input and output ports is restored from the blocked state, by making each movable mirror to be in an operating state at a desired angle to which the above offset portion is added, a normal path is connected. Further, when the path is switched, the connection to another port is performed, by switching the operation of each movable mirror to a current angle and adding an angle according to an arrangement of pre-stage and latter-stage ports.
However, in the case where a large number of input and output ports is disposed to the conventional wavelength selective switch as described above, by merely offsetting the angle of each movable mirror to the Y-direction as shown in FIG. 14 and FIG. 15, it becomes hard to reliably block the connection between the input and output ports at the non-operating time. To be specific, in a wavelength selective switch including four input ports Pin1 to Pin4 and one output port Pout as shown in FIG. 16 for example, it is necessary to determine the angle of each movable mirror at the non-operating time so that the optical coupling is equal to or less than an allowable value between all the input ports and the output port inclusive of the return to the input ports. Therefore, there is caused a problem in that it becomes impossible to sufficiently respond to combinations of large number of input and output ports by merely offsetting the angle of each movable mirror to only the Y-direction.
To solve the above problem, as shown in a top view of FIG. 17 for example, the angle of each movable mirror is offset to the X-direction (angular dispersion direction in the diffraction grating), and the lights which are reflected by the respective movable mirrors and thereafter reach the collimator array are deviated not to the X-direction but to the Y-direction, so that the connection between all the input and output ports can be blocked at the non-operating time.
However, in the case where the angle of each movable mirror is offset to the X-direction as described in the above, as shown in a lower stage of FIG. 17, when the input and output ports are connected at an operating time, since distances of a focal lens to the movable mirrors corresponding to the respective wavelengths on the mirror array 104 are different from each other, beam diameters of the lights condensed on the reflecting surfaces of the movable mirrors are different for each wavelength. Therefore, there are caused problems in that a loss difference occurs between the lights of respective wavelengths, which are reflected by the respective movable mirrors to be led to the output port, and also, a beam of the light condensed on the reflecting surface of each movable mirror is spread so that a wavelength band of the light becomes narrower.